Underwater granulation or pelletizing has proven to be particularly effective for the granulation of thermoplastic materials and is utilized worldwide for the industrial manufacture of plastic granulate (e.g. DE-OS 26 46 309 and its equivalent U.S. Pat. No. 4,150,595). Here, the hot plastic melt coming from an extruder directly enters a water bath in a granulator housing through the outlets or nozzles of an orifice plate. The strands of plastic from the outlets are cut, by a cutting device adjacent to the orifice plate, into granules which are washed away with cooling water above the housing and are dried in a post-treatment station. The start-up of such devices often presents difficulties, since during start-up, i.e., upon introduction of the cooling water into the granulator housing to fill the same, the melt outlets in the orifice plate can become partially or completely clogged due to solidification of the melt therein, particularly since in this stage the melt throughput is still very low and thus lacks the heating power of the full flowing hot melt.
In order to alleviate this deficiency, it is known from DE-OS 19 37 862 to arrange a start-up valve between the extruder and the orifice plate in order to divert the product from the extruder into the atmosphere for a period of time according to the individual circumstances. The use of such a start-up valve in fact provides improvement in the start-up process as regards supply of sufficiently heated plastic melt to the orifice plate, but the solidification in individual melt outlets cannot be completely eliminated due to the input of water into the granulator housing.
Water ring granulation is disclosed in DE-PS 14 54 888 and its equivalent U.S. Pat. No. 3,343,213 as an alternative to underwater granulation and therein the cutting device is surrounded by a granulate collecting and cooling housing and the granulate is spun from the cutting device to the water ring, which is formed on the inside wall of the housing. Water ring granulation cannot be employed for all types of thermoplastic materials, particularly those which tend to adhere to the orifice plate and to the knives of the cutting device.